Heretofore, a method for selectively separating carbon dioxide has widely been studied because of its wide application range. For example, purity of hydrogen can be improved by selectively separating carbon dioxide from a reformed gas for a fuel cell. Further, it is expected that the progression of global warming may be suppressed by selectively separating carbon dioxide which is one of causes of global warming, and storing the separated carbon dioxide on the sea bottom.
Looking at the hydrogen production process, in a reforming system for a hydrogen station, which is currently been developed, hydrogen is produced by reforming hydrocarbon into hydrogen and carbon monoxide (CO) through steam reforming, and reacting carbon monoxide with steam using a CO shift reaction.
In a conventional CO shift reactor, the cause for inhibition of miniaturization and reduction of the start-up time is considered that a large amount of a CO shift catalyst is necessary because of the restriction on chemical equilibrium of the CO shift reaction represented by (Chemical Formula 1) shown below. For example, 20 L of a reforming catalyst is required in a 50 kW reforming system for phosphoric acid fuel cell (PAFC), whereas, an about 4-fold amount (77 L) of a CO shift catalyst is required. This is a large factor, which inhibits miniaturization of the CO shift reactor and reduction of the start-up time. The symbol  means a reversible reaction.CO+H2OCO2+H2  Chemical Formula 1
Therefore, when a CO shift reactor is equipped with a CO2-facilitated transport membrane capable of being selectively permeated by carbon dioxide and when carbon dioxide at the right side produced by the CO shift reaction of Chemical Formula 1 shown above is efficiently removed out of the CO shift reactor, chemical equilibrium can be shifted to the hydrogen production side (right side) to obtain a high conversion ratio at the same reaction temperature, thus making it possible to remove carbon monoxide and carbon dioxide over limitation due to the restriction of equilibrium. This state is schematically shown in FIGS. 20 and 21. FIGS. 21A and 21B respectively show each change in the concentration of carbon monoxide and carbon dioxide along the catalyst layer length of the CO shift reactor in the case where the CO shift reactor is equipped with a CO2-facilitated transport membrane or not.
Since the above CO shift reactor (CO2 permeable membrane reactor) equipped with a CO2-facilitated transport membrane enables removal of carbon monoxide and carbon dioxide over limitation due to the restriction of equilibrium, it is possible to reduce a load of pressure swing adsorption (PSA) of a hydrogen station and to lower S/C (steam/carbon ratio) of the reforming reaction and CO shift, thus making it possible to reduce the cost of the entire hydrogen station and increase efficiency. Since higher performances (increase in SV) of the CO shift reaction can be achieved by being equipped with the CO2-facilitated transport membrane, miniaturization of the reforming system and reduction of the start-up time can be achieved.
Example of the related art of the CO2 permeable membrane reactor is disclosed in Patent Document 1 (or Patent Document 2 having the same contents published by the same inventors).
The reforming system proposed in patent Documents 1 and 2 provides a CO2-facilitated transport membrane process which is useful for purification and water gas shift reaction (CO shift reaction) of a reformed gas generated when fuels such as hydrocarbon and methanol are reformed into hydrogen for a fuel cell vehicle on the vehicle, and typical four kinds of processes are disclosed in the same Patent Documents. When hydrocarbon (containing methane) is used as a raw material, by selectively removing carbon dioxide using a membrane reactor in which a water gas shifter (CO shift reactor) is equipped with a CO2-facilitated transport membrane, the reaction rate of carbon monoxide is increased and the concentration of carbon monoxide is decreased, and also purity of hydrogen thus produced is increased. Further, percentage-order carbon monoxide and carbon dioxide remaining in hydrogen produced are reacted with hydrogen in a methanator thereby converting into methane, and thus the concentrations are decreased and a decrease in efficiency of a fuel cell due to poisoning is prevented.
In patent Documents 1 and 2, as the CO2-facilitated transport membrane, a hydrophilic polymer membrane of PVA (polyvinyl alcohol) containing mainly a halogenated quaternary ammonium salt ((R)4N+X−) as a carbon dioxide carrier is used. Example 6 of Patent Documents 1 and 2 discloses a method for producing a CO2-facilitated transport membrane formed of a composite membrane of 50% by weight of a 49-μm thick PVA membrane containing 50% by weight of a tetramethylammonium fluoride salt as a carbon dioxide carrier, and a porous PTFE (polytetrafluoroethylene) membrane which supports the PVA membrane, and Example 7 discloses membrane performances of the CO2-facilitated transport membrane when a mixed gas (25% CO2, 75% H2) is treated under a total pressure of 3 atm at 23° C. Regarding the membrane performances, CO2 permeance RCO2 is 7.2 GPU (=2.4×10−6 mol/(m2·s·Pa)) and CO2/H2 selectivity is 19.
Patent Document 3 shown below discloses, as a CO2-facilitated transport membrane, a CO2 absorbent formed by cesium carbonate in combination with amino acid.
The method for producing a CO2-facilitated transport membrane described in Patent Document 3 is as follows. First, a commercially available amino acid is added to an aqueous solution of cesium carbonate so as to obtain a predetermined concentration, followed by well stirring to prepare an aqueous mixed solution. A gel-coated surface of a gel-coated porous PTFE membrane (47Φ) is then immersed in the prepared mixed solution for 30 minutes or more, and the membrane is slowly pulled up. A silicone membrane is placed on a sintered metal (for the purpose of preventing the permeation side from being wetted with the solution) and the above hydrogel membrane (47 mmΦ) is placed thereon, followed by sealing through covering with a cell with a silicone packing. A feed gas is allowed to flow at a rate of 50 cc/min over the CO2-facilitated transport membrane thus produced, and the pressure of the lower side of the membrane is reduced to about 40 torr by evacuating the lower side.
In Example 4 of Patent Document 3, when a CO2-facilitated transport membrane formed by cesium carbonate and 2,3-diaminopropionic acid hydrochloride at each molar concentration of 4 (mol/kg) is used, a CO2 permeation rate is 1.1 (10−4 cm3(STP)/cm2·s·cmHg) and a CO2/N2 separation factor is 300 under the temperature of 25° C. Since the CO2 permeance RCO2 is defined by a permeation rate per pressure difference, the CO2 permeance RCO2 in Example 4 of Patent Document 3 is calculated as 110 GPU. However, data with respect to CO2/H2 selectivity in the present Example is not disclosed.
Patent Document 4 shown below discloses a CO2 separation membrane formed of a cellulose acetate membrane containing an alkali bicarbonate added therein. However, Patent Document 4 describes only about CO2/O2 selectivity and does not disclose data about CO2/H2 selectivity. Furthermore, the disclosed data are measured under the conditions of a low pressure (about 0.01 atm) and the data measured under the pressure condition of about several atm are not disclosed.
Patent Document 1: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2001-511430
Patent Document 2: Specification of U.S. Pat. No. 6,579,331
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2000-229219
Patent Document 4: Specification of U.S. Pat. No. 3,396,510